We had slightly different readings.
As he was writing he became
aware that he was being watched, and a figure slowly emerged to his left. It was
indistinct and on the periphery of his vision but it moved as V.T. would expect a
person to. The apparition was grey and made no sound... V.T. was unable to see any detail and
finally built up the courage to turn and face the thing. As he turned the apparition
faded and disappeared.
He experienced a visual disturbance in his periphery manifesting as the false perception of a person. Even without it being interpreted as a person, that's a textbook mild hallucination.
Once V.T. knew this he calculated the frequency of the standing sound
wave ... 18.97Hz ... plus or minus 10%
Table IV on page 212 of this book shows frequencies causing disturbance to the eyes
and vision to be within the band 12 to 27 Hz.
Most interestingly, a NASA technical report
mentions a resonant frequency for the eye as 18 Hz (NASA Technical Report
19770013810).
He cited two sources inline with ranges narrower than 8-40Hz which indicate that vision can be affected at the same frequencies he measured in the lab. He even noted that everyone would have slightly different resonant frequencies.
No, it's not a full research paper, but it is the citation you requested.